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LeetCode 378 Kth Smallest Element In A Sorted Matrix - Medium

378 Kth Smallest Element in a Sorted Matrix

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378. Kth Smallest Element In A Sorted Matrix — Medium

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Problem

378 Kth Smallest Element in a Sorted Matrix

Given a n x n matrix where each of the rows and columns are sorted in ascending order, find the kth smallest element in the matrix.

Note that it is the kth smallest element in the sorted order, not the kth distinct element.

Example:

matrix = [ [ 1, 5, 9], [10, 11, 13], [12, 13, 15] ], k = 8,

return 13. Note: You may assume k is always valid, 1 ≤ k ≤ n2.

Solution

# Binary search, bisect.bisect_right returns the number of the elements less or equal to the target
class Solution:
    def kthSmallest(self, matrix: List[List[int]], k: int) -> int:
        lo, hi = matrix[0][0], matrix[-1][-1]
        while lo < hi:
            mid = (lo + hi) // 2
            temp = 0
            for row in matrix:
                temp += bisect.bisect_right(row, mid)
            if temp < k:
                lo = mid + 1
            else:
                hi = mid
        return lo

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